[InstCombine] allow undef elements when comparing vector constants for min/max bailout

This is a hacky, but low-risk fix to avoid the infinite loop in PR46271:
https://bugs.llvm.org/show_bug.cgi?id=46271

As discussed there, the problem is that FoldOpIntoSelect() can get into a conflict
with a transform that wants to pull a 'not' op through min/max via
SimplifyDemandedVectorElts(). We need to relax our matching of min/max to include
undefined elements in vector constants to avoid that. Alternatively, we could
improve or cripple the demanded elements analysis, but that could create even
more problems.

The likely better, safer alternative will be to create min/max intrinsics, so
we can remove all of the hacks related to min/max matching in instcombine.

Differential Revision: https://reviews.llvm.org/D81698